Around 73,000 years ago, the towering predecessor of the Fogo volcano - one of the most active in the world - collapsed. As a result, a unbelievably massive tsunami rippled across the Atlantic Ocean, washing its destructive force over islands that now boast over 250,000 human residents. Experts now wonder if such a disaster is more common than we'd like.

Even today, post-collapse Fogo remains some 2,829 meters (9,300 feet) above sea level, and erupts about every 20 years -- most recently last fall. This would hint that ancient Fogo was massive, and its collapse resulted in crashing waves equally towering.

That, as it turns out, is exactly what scientists working off the west African coast in the Cape Verde Islands have found. Several years ago Ricardo Ramalho, a researcher with Columbia University's Lamont-Doherty Earth Observatory, spotted unusual boulders lying as far as 2,000 feet inland and nearly 650 feet above sea level. They were kind of hard to miss -- big as delivery vans and completely out of place, lying on volcanic ground where limestone and basalt boulders do not belong. (Scroll to read on...)

After considering every possible scenario, Ramalho and his colleagues concluded that these boulders must have come from groups of similar marine-side rocks that ring the island's shoreline and and were thrown onto notably younger volcanic terrain via wave.

Amazingly, that would mean a wave somehow carried rocks that weighed up to 770 tons. The researchers estimated that it would take a gargantuan wave some 800 feet high to hold the boulders aloft. The results were published in the journal Science Advances.

What's most interesting about this study, however, is not the revelation that a breathtakingly massive tsunami once rippled across a corner of the globe (although, that's pretty darn cool on its own). Ramalho's work may actually revive a simmering controversy among geoscientists over whether sudden volcanic collapses present a realistic hazard even today. Should we expect other megatsunamis in the future?

"Our point is that flank collapses can happen extremely fast and catastrophically, and therefore are capable of triggering giant tsunamis," Ramalho explained in a statement. "They probably don't happen very often. But we need to take this into account when we think about the hazard potential of these kinds of volcanic features." (Scroll to read on...)

There have been some amazing megatsunami events in our time. In 1958, for instance, an astounding 1,724-foot-high wave -- the largest ever recorded -- washed across a corner of Alaska. Two fishermen and their boat were reportedly held aloft for the whole event. Not only did they clear an entire forest while atop the wave, but miraculously lived to tell the tale.

"People shake their heads when I tell them I saw it that night," one eyewitness said, describing how a nearby glacier rose several hundred feet in the water's swell. "I can't help it if they don't believe me."

However, that event resulted from a combined earthquake and 90 million ton rockslide crashing into the state's isolated Lituya Bay. Underwater volcanoes in the middle of an ocean - the true concern of Ramalho and his colleagues - are much more complicated. (Scroll to read on...)

The controversy partially hangs on the physics of waves. In the case of the Fogo collapse, that 800-foot wave would have had to travel 34 miles (55km) to reach Cape Verde. It has been argued that in the open ocean, waves created by geological collapse quickly lose energy simply because of all the surface area their momentum has to cover.

Bill McGuire, a tsunami experts with the University College London who was not involved in the research, said the study "provides robust evidence of megatsunami formation [and] confirms that when volcanoes collapse, they can do so extremely rapidly."

Based on his own work, McGuire s says that such megatsunamis probably come only once every 10,000 years. "Nonetheless," he added, "the scale of such events, as the Fogo study testifies, and their potentially devastating impact, makes them a clear and serious hazard in ocean basins that host active volcanoes."

Ramalho himself cautions against jumping to conclusions, adding that the Fogo event "doesn't mean every collapse happens catastrophically."

"But," he added, "it's maybe not as rare as we thought."

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